Problem: The scale on a map is 4cm : 2km. If two cities are 16cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 4cm on the map corresponds to an actual distance of 2km. A distance of 16cm on the map is the same as 4 $\cdot$ 4cm. The actual distance between the two cities is 4 $\cdot$ 2km, or 8km.